WASHINGTON (AP) _ A study by the National Academy of Sciences says there is no evidence that genetically engineered crops have harmed the environment but faults the government for failing to monitor many crops after they are approved for commercial use.

``Without systematic monitoring, the lack of evidence of damage is not necessarily lack of damage,'' the study says.

The study, released Thursday, also says the Agriculture Department needs to seek more advice from scientists outside the agency when changing regulations and do a better job of getting comment from the public.

USDA, which requested the study after critics accused it of lax regulation, is supposed to ensure that hardier, gene-altered crops don't develop into superweeds or endanger insects and other animals.

Fred Gould, a North Carolina State University scientist who led the study, said the problems it cited amounted to ``small loopholes.''

``We are offering suggestions for a system that is functioning. We're not condemning the system,'' Gould said.

Although the biotechnology industry is still in its infancy, genetically engineered corn and soybeans are now being used widely by farmers.

Under the Agriculture Department's all-or-nothing approval process, biotech crops that are deemed to be safe for the environment can generally be grown anywhere in any amount that farmers choose.

USDA officials say they have no legal authority to do anything else. Crops that produce their own pesticides are regulated more stringently, because they are handled by the Environmental Protection Agency under a different law.

The study also said the Agriculture Department allows biotech companies to keep too much of their research data from the public by classifying the material as business secrets. The same data are sometimes made public in Canada and Europe, the study said.

Jane Rissler, a critic of the biotechnology industry with the Union of Concerned Scientists, said the study shows that USDA oversight of gene-altered crops is inadequate.

``It is not a difficult bar to meet to get USDA approval for a genetically engineered crop,'' she said.